A velocimeter measures the velocity of a target structure or object. A given target for which a velocity measurement is desired can be in the form of a solid or fluid. As the structural models used to represent targets become more detailed, there is an increasing need to measure the velocities of small components in microsystems, for example, micro-electro-mechanical systems, or MEMS. In the MEMS example, it is desirable to measure the velocity of parts such as gears and accelerometer reaction masses. It is not practical to attach a transducer to small components such as these, because the transducer will change the structure significantly due to mass loading by the transducer itself. Therefore, a non-contact method of velocity measurement is needed. There is a similarly increasing need to measure velocities associated with, for example, rotations, vibrations and impacts in macroscopic systems of larger components.
A laser Doppler velocimeter (LDV) is an interferometric instrument that uses the Doppler Effect to measure velocity. Laser light from a source at a single optical frequency is scattered by a moving object (e.g., an extended solid, particles, or even gases), and the optical frequency of the laser light is shifted due to the motion of the object. The optical frequency of the Doppler shifted light is compared to the aforementioned optical frequency of the laser light source. The difference between these two optical frequencies is proportional to the velocity of the target. With typical velocities involved in structural mechanics, the difference frequency can be expected to be at least in the tens of kilohertz to tens of megahertz (MHz). Frequencies in this range typically limit conventional LDVs to performing a single point velocity measurement at a time. A set of velocities associated with an array of physical points on the target is referred to as a “velocity image” of the target. Such a velocity image is conventionally built up by obtaining velocity information for a plurality of different points on the target in sequential fashion, one point after another. This can take several minutes for a velocity image of an object in steady state vibration, and is impossible for transient events such as impacts.
Some conventional approaches propose to produce a velocity image without the aforementioned temporal limitation that sequential, point-after-point operations impose. They physically replicate a single point instrument as many times as necessary to produce a composite instrument that contains enough single point instruments operating in parallel to generate the desired velocity image all at once. This physical replication approach is essentially a “brute force” approach which requires additional expense and design complexity.
It is desirable in view of the foregoing to provide the capability of producing a velocity image without the aforementioned difficulties associated with prior art approaches.
Exemplary embodiments of the present invention heterodyne the incoming Doppler-shifted beams to reduce their frequencies into the bandwidth of a digital camera. This permits the digital camera to produce at every sampling interval a complete two-dimensional array of pixel values. This sequence of pixel value arrays provides a velocity image of the target.